Page 32 - Design of Simple and Robust Process Plants

1.8 Design of Instrumentation, Automation and Control 15
1.7
Optimization of a Complex, and Evaluation of its Vulnerability

The optimization of a complex ± which might be a chemical complex, a refinery or a
number of food processing plants sited at one location ± will be discussed in Chapter 7.
The investments in logistic facilities and services at a complex are very high. The design
philosophies for a complex of processes are presented, and on the basis of these a quan-
titative methodology based on reliability engineering techniques is shown to optimize
the logistics of a complex. The vulnerability of a complex can be quantified through the
development of a reliability flowsheet of its different sections/processes, including uti-
lity generation and external supplies. The vulnerability of a complex can be evaluated
and its potential losses quantified and compared with alternatives, which include back-
up provisions, sizes of storage, or improved reliability of units.

1.8
Design of Instrumentation, Automation and Control

The design of the automation and control are essential elements to comply with a
process that is operated hands-off, and under the most economical conditions. To
realize this objective, attention is given in Chapter 8 to the design of all the elements
that contribute to it. An empirical approach will not be sufficient to achieve the level
of robustness; rather, a dynamic simulation will need to be made available in order
to enable the design.
The instruments are the eyes, ears, and hands of the process, and special empha-
sis must be given to them. Firstly, they need to be measuring correctly what we
want to measure in terms of the process condition. This seems straightforward, but
how often do we measure a volumetric flow that is influenced by density or temper-
ature when we are really interested in a mass flow? Other important aspects of
instruments include: range, accuracy, reliability, robustness, in-line measurement
(avoid sample lines) self-diagnostics, installation, and calibration. All measure-
ments should provide the correct information so that the process's organs of sense
ªseeº what is happening and automation and control systems can take the correct
action.
Nowadays, automation of operation is making extensive progress in terms of
implementation. As automated process become more consistent in executing
tasks, the result in turn is more consistent production. However, a major question
to be addressed is, ªhow far do we want to go with automation with regard to the
role of the operator?º How do we keep the operator alert and responsive during
the operation? Currently, a number of quantitative investigations are under way to
cover this point. However, one important aspect of simple designed process is that,
by increasing the level of automation, the DOFs of the operator are not reduced
and the system made less complex from operational perspective, (Chapter 2). This
assumes that the operator does not have all kinds of manual over-rides. The sur-
veyability of the process is another important aspect for a good man±machine